1. Field of the Invention
The present invention relates to a molding method of a thermosetting resin in which molding is carried out by pouring or injecting a thermosetting resin into a mold and then heating the same, a mold for the molding and a molded article.
2. Related Background Art
In a conventional molding of a thermosetting resin, an extra portion is usually attached to a molded article due to the leakage of the resin in the case of a molding method in which the thermosetting resin is poured or injected into a mold. For the molded article with the extra portion (hereinafter referred to as xe2x80x9cflashxe2x80x9d) due to the leakage of the resin, a step of deflashing is required after the molded article has been taken out of the mold.
In recent years, injection molding has often been used, which permitted the molding in a short period of time. Most of all, however, among integral molding techniques, an insert molding method is utilized, in which parts of a metal or the like are placed in the mold, and then the resin is injected into the mold to obtain an integrally molded article.
In such a molding technique, in order to omit the deflashing step after the molding, various methods for reducing the flash at the molding have been suggested. In such methods, however, the mold having a very high precision is required, and in the case of the insert molding, the parts to be inserted must have a very high dimensional accuracy. In addition, it is necessary to strictly control the amount of the resin that is poured or injected into the mold, which makes the molding itself very difficult.
In the injection molding of the thermosetting resin, the mold to be used is manufactured with a very high dimensional accuracy on the basis of a calculation involving a thermal expansion of a mold material, in view of the fact that the temperature of the mold rises to a high level due to the setting of the resin and most of the thermosetting resins are liquid before the setting.
However, even if it is attempted to manufacture the mold with the high precision, the working of the mold is limited, and it is impossible to have an error of zero. Usually, there is an error of about 0.01 mm or less.
For example, an insert is usually manufactured by pressing a metallic plate to blank it, but at the time of the blanking, a step (or a difference in level) is formed between the flash side and the sag side of the surface of the blanked plate, and this step causes the flash.
For example, a zinc-coated steel plate, which is a metallic steel plate, is blanked by the use of a blanking mold that is 0.06 to 0.07 mm smaller than a predetermined size, but at this time, a difference in level (unevenness on a blanked section formed by the blanking) is formed between the blanked side and the flash side. The softer the material of the mold is, the larger this step is, and usually, when the soft material is used, the step further increases as much as about 0.02 to 0.03 mm.
As described above, the mold has a tolerance of 0.01 mm or less as its precision, and the metallic plate to be inserted, e.g., the above zinc-coated steel plate, also has a size tolerance of 0.06 to 0.07 mm. In consequence, a gap of about 0.07 mm is always present between the mold and the insert.
For example, in the injection molding of a silicone rubber, which is one of the thermosetting resins having a low viscosity, the molding is possible even at the tip of the gap of 0.07 mm.
In the general injection molding, the molding is carried out at a high speed and under a high pressure, so that a large amount of the resin inconveniently leaks out. That is to say, if it is attempted to prevent the leakage of the resin, the pressure cannot be applied to the mold, so that fine air bubbles remain in the resin article, resulting in a poor function of the article.
Furthermore, because of the leakage of the resin, the flash may stop an automatic conveyor line, which may lead to the drop in operating efficiency. Moreover, the deflashing process is naturally required, and it is very difficult to remove a very small flash by an automatic device, and hence, cost noticeably increases. In addition, since the thermosetting resin, which cannot be effectively recycled, is used, the amount of waste materials increases. Thus, injection molding increases cost and causes environmental problems.
Accordingly, the present invention has been developed in view of the above problems, and its object is to provide a molding method of a thermosetting resin in which the formation of a flash can be prevented, a mold for the molding, and a molded article.
In order to solve the above problems and to achieve the above object, the present invention is directed to a molding method of a thermosetting resin, which comprises pouring or injecting the thermosetting resin into a mold, and then heating and molding it. This method comprises the step of locally heating an unnecessary gap and its peripheries of the mold, which are not concerned with the molding of the thermosetting resin into a desired article having a required shape, whereby the resin in the gap and its peripheries is promptly set to reduce the leakage of the resin into the unnecessary gap.
That is to say, according to the present invention, when a thermosetting resin is molded, a heat source is disposed at a resin leakage portion in a mold to promptly heat its resin leakage portion alone, whereby the resin in the resin leakage portion is promptly set to form a packing of the set resin, so that the leakage of the resin can be prevented. As a result, a molded article of the thermosetting resin free from flash can be produced.